From Wikipedia:
The story opens when mousy Orfamay Quest walks into Philip Marlowe's office in search of a detective. Orfamay is a "small, neat, rather prissy-looking girl with primly smooth brown hair and rimless glasses" who has recently moved to Bay City from Manhattan, Kansas to search for her older brother Orrin. Orrin had recently come out to Bay City to work as an engineer for the Cal-Western Aircraft Company, but has since then stopped writing to Orfamay and their mother. Orfamay describes her concern to Marlowe and asks that he find the whereabouts of her brother, despite giving Marlowe few leads with which to work…http://en.wikipedia.org/wiki/The_Little_Sister